1. Field of the Invention
The present invention relates to a power amplifier, and more specifically, to an apparatus and method for calibrating the center frequency of a power amplifier.
2. Description of the Prior Art
For modern integrated circuits (ICs), power amplifiers are applied extensively in wired and wireless communication devices. Please refer to FIG. 1 in conjunction with FIG. 2. FIG. 1 is a circuit diagram of a conventional power amplifier 100. FIG. 2 is a frequency response diagram of the power amplifier 100 shown in FIG. 1. The power amplifier 100 includes a loading circuit 120 and an amplifying circuit 140 for generating an output signal Vout based on an input signal Vin. According to the frequency response of the power amplifier 100, the gain of the power amplifier 100 is controlled by its center frequency, usually determined according to capacitance and inductance of the loading circuit 120. As shown in FIG. 1, the loading circuit 120 comprises a capacitor 122 (the capacitance is C0) and an inductor 124 (the inductance is L0) connected in parallel with the capacitor 122. The capacitance C0 and the inductance L0 both determine the center frequency Fc of the frequency response shown in FIG. 2. The equation showing this relationship is:
                    Fc        =                  1                      2            ⁢            π            ⁢                                                            L                  0                                ×                                  C                  0                                                                                        eq        .                                  ⁢                  (          1          )                    
It is known that unexpected factors in the manufacturing process can cause the capacitance C0 of the capacitor 122 and the inductance L0 of the inductor 124 in the loading circuit 120 to stray from desired values, therefore the center frequency Fc can also stray from a predetermined frequency, as can be observed from eq.(1). Moreover, other factors, such as operating temperature of the power amplifier 100, result in a shift of the center frequency Fc. Please refer to FIG. 2. Assuming the ideal center frequency of the power amplifier 100 is Fc and given an input signal Vin with frequency Fc, the power amplifier 100 amplifies the input signal Vin to generate an output signal Vout corresponding to a gain A at frequency Fc. However, if component characteristics in the loading circuit 120 have some alterations, the center frequency will shift from Fc to Fc′. In this condition, given an input signal Vin with frequency Fc, the power amplifier will amplify the input signal Vin to generate another output signal Vout corresponding to another gain A′ at frequency Fc. Obviously, as the gain A′ is smaller than the gain A and the center frequency of the power amplifier 100 strays, the output power is decreased. As a result, not only is the transmission quality decreased but the transmission distance is shortened; seriously affecting the signal transmission.